A great deal goes into not only finding a new fish species, but in confirming that it is, in fact, unknown to science and then placing it on the “tree of life”. The scientific names and classifications of fishes – and all living things – are based on specimens that are collected in the field and preserved in natural history collections like the Academy of Natural Sciences here in Philadelphia. Published literature creates a verifiable chain of custody that ties taxonomic names to their authors, dates, usage and museum specimens.
Mark Sabaj Perez, PhD, collection manager of fishes at the Academy of the Natural Sciences, co-authored a paper in Molecular Phylogenetics and Evolution which proposes a “tree of life” for South American knifefishes. This group includes the electric eel and related fishes with blade-shaped bodies. He spoke with the Drexel News Blog to explain some of his findings, including how classifications are subject to change and, in certain ways, are subjective.
What are knifefishes and what makes them so interesting?
Neotropical knifefishes include about 280 species in the order Gymnotiformes – a taxonomic group that is above the rank of family and below the rank of class in the classification of living organisms, past and present. The order Gymnotiformes is related to Characiformes (tetras, piranhas, pacus, etc.), Cypriniformes (carps and minnows), Siluriformes (catfishes), and the recently proposed Cithariniformes, an African group that doesn’t really have a common name. Together, these five fish orders dominate nearly all freshwaters on Earth. Gymnotiforms are found in all sorts of freshwater habitats from southern Mexico to central Argentina. Some only occur in the deep dark channels of large rivers, like the Amazon and Orinoco. Others are restricted to small creeks and lowland swamps. All gymnotiforms are capable of some degree of electrogenesis and electroreception. The one that most people already know is the electric eel, Electrophorus electricus – which is not an eel, but a knifefish. Electric eels have fascinated people for a long time due to their ability to generate electricity: up to 860 volts as recorded by Dr. William Crampton, a co-author on this study. By comparison, wall outlets in the United States generally run at only 120 volts.
How do electric “eels” generate electricity?
Its long, cylindrical body, which can reach 2.5 meters and weigh nearly 45 pounds, is mostly made up of specialized muscle cells arranged in vertical columns (stacks) and longitudinal rows. The movement of salt and potassium ions across these battery-like cells generates electricity. The “eels” use this electricity for communication and navigation, hunting and stunning prey, and to remind predators (and ichthyologists) to mind their own business.
Have you encountered electric eels in the wild?
Yes – on many occasions! Getting shocked by an electric eel is a rite of passage for those who study them. My most intense experience was during an expedition to the Xingu River in Brazil. Our local guides found a clear shallow pool full of eels. Perched on logs, the guides used homemade gaffs to spear the eels as they moved through the submerged brush lining the pool. After landing a few, they asked if I wanted a try – but by then the water was too murky to spot the eels. So, I was instructed to stand in the middle of the muddy pool, watch for an eel to break the surface to gulp air (eels get oxygen from air, not water, like most fishes) – and then lunge at it with the gaff. I managed to catch one this way – but not without receiving a tremendous shock and adrenaline spike that I will never forget. The guides had a good laugh at my electric boogaloo.
How many of species of electric eels are there and what are their names?
Until recently, ichthyologists recognized only one species, Electrophorus electricus. In 2019, Will Crampton and I were part of a large international team that proposed two new species of electric eels, Electrophorus voltai and Electrophorus varii. The first honors Alessandro Volta (1745–1827), who developed the first electric battery based on his study of eel anatomy. The second was named for a beloved colleague at the Smithsonian Institution, Richard Vari, who had passed away in 2016. So there are now three species of electric eels – but their names depend on who you ask. Back in 1941, Shoji Nakashima, a Japanese ichthyologist, proposed a new species from Peru and named it Electrophorus multivalvulus. Since then, most considered it a synonym of Electrophorus electricus, proposed by Carl Linnaeus back in 1766. When two names apply to the same species, the older name is used.
Electrophorus electricus is the oldest name – so, that will always be used for one species. Electrophorus voltai is a solid name for the electric eels found in rivers draining the Brazilian Shield. That is to say, there are no older names for this distinct species. But, by recent accounts (including our study), Electrophorus varii appears to be the same species as Nakashima’s – in which case his multivalvulus prevails as the older name.
Who makes the final decision on what species names are officially used?
That’s a good question. There is no money at stake – like when a football stadium is named (or renamed). But, important information on a particular species (distribution, diet, protected status, etc.) is tied to its proper scientific name. In a similar sense, a vehicle’s information is tied to its make and model (e.g., Toyota RAV4) or the information on a lake is tied to its proper geographic name (e.g., Erie). For now, we consider Electrophorus multivalvulus to be the proper name for the species of electric eel that occurs throughout most of the Amazon Basin. But a future study might show that eels from the Peruvian Amazon are different enough from those in the Brazilian Amazon to be considered separate species. In that case, multivalvulus would apply to the Peruvian species and varii would apply to the Brazilian one…unless someone uncovers an older applicable name for one or the other.
What new insights did you discover it your latest study about knifefishes?
The current study, led by Dr. Francesco Janzen, uses molecular data (DNA sequences) to propose a new hypothesis for how the species of Neotropical knifefishes are related to one other. Those evolutionary relationships are represented by a phylogeny – or tree-like diagram where two (or more) branches can be traced back in time to a common ancestor. That phylogeny, in turn, informs our new classification. The molecular data for this study was/were collected by Dr. Janzen in Dr. Nathan Lovejoy’s lab at the University of Toronto.
Can you explain the process behind the practice of upgrading and downgrading names from one rank to another?
This practice for ranking taxonomic names is partly subjective. Generally, it’s based on an understanding of relationships within a particular group – in this case, Neotropical knifefishes – and their broader relationships to other groups, the Cypriniformes, Siluriformes, and other “-formes” mentioned earlier. Those relationships, in turn, are based on analyses of data taken on specimens, like those preserved at the Academy of Natural Sciences. That data may refer to anatomical features, or DNA sequences, or other heritable traits. Phylogenetic analyses determine which traits support which groups at what level. Ideally, the classification is a summary of the group’s evolutionary history – as in, all species in a particular genus are descended from a common ancestor, and so more closely related to each other than to species in other genera. The same idea should hold true for higher rankings – Tribe, Subfamily, Family, Superfamily, etc.
In summary, is your hypothesis that these three species of electric eels, deserve recognition as a separate “family?”
Yes. We argue that because of their unique anatomy, physiology, ecology and genetics, the electric eels deserve to be in their own distinct family, Electrophoridae. According to Eschmeyer’s Catalog of Fishes, this family was recognized as far back as 1872 – but was downgraded to subfamily in 2001 (hence the “new”). Ichthyologists are free to accept or reject this revised (and partly subjective) ranking. As for our new phylogeny – how these knifefishes are related to one another – that part is objective, an evidence-based hypothesis to be tested. Future analyses will either support, reject, or tweak our new hypothesis.
Reporters interested in speaking with Sabaj Perez should contact Mike Tuberosa, assistant director, News & Media Relations, at mt85@drexel.edu or 215.895.2705.
